As one of the most common onion varieties, these European alliums are a must for Northern growers. Like other onions and bulb plants, yellow Spanish onions prefer cool weather, lots of direct sunlight and rich, well-draining soil that is slightly acidic. Provide about an inch of water per week, but never let the soil get soggy, as the bulbs will rot. Conversely, soil that is too dry causes bulb splitting. Light mulching is the easiest way to preserve moisture and limit weed growth.

Watch the leaves for signs of pest infestations, which include thrips and onion maggots. Along with rotting, blight, downy mildew and purple blotching are potential diseases. Immediately remove infected bulbs and take steps to protect the remaining crop.

Because of the long growing season, starting Spanish onions indoors is encouraged. Sow the seeds in a good seed-starting mix 8-12 weeks before the last expected spring frost. Use seed trays or small pots, lightly covering the seeds with soil. Add vermiculite and/or a humidity dome for moisture retention. Mist or bottom-water the soil as needed, maintain temperatures between 60 and 75 degrees and use grow lights to provide 16 hours of light per day. Transplant the seeds 4-6 weeks before the final frost, hardening them off for a week beforehand.

You can plant onions outdoors once the soil is workable. Space your seeds or seedlings 4-6 inches apart. Never plant alliums in the same spot back-to-back years, as this increases disease risk. As a long-day allium, Spanish onions only form bulbs when the sun is out for 14-16 hours per day. As such, you can’t grow them as a winter crop even in mild climates.

For container planting, use large pots that will fit the onions when they are mature so repotting isn’t necessary. Use a vegetable-formulated potting mix and add sand or perlite to help with drainage.

Sweet Spanish onions are heavy feeders, so they need consistent nutrition. Start fertilizing seedlings and container onions at the four-week mark, using liquid fertilizer every 7-10 days. For in-ground onions, add balanced fertilizer or compost to the soil when you plant. Then apply a nitrogen-rich fertilizer every 3-4 weeks; fish emulsions are a fine pick for organic growers. Stunted growth and pale leaves are signs of inadequate fertilization, which you can confirm with soil testing.

Overwintering isn’t necessary since these onions are grown as annuals. Remove all bulbs from the ground before snowfall and let the onions cure in a warm, well-ventilated area for three weeks. Then store them in a cool, dry and dark space using crates or mesh bags.

A little pruning goes a long way. When the plants have developed two sets of true leaves, cut the tops back to 2-3 inches tall to help them grow straighter. In the late stages, gently bend the tops to alert the plant to shift to bulb production.

Yellow Spanish onions can take 3-4 months to fully mature. The bulbs are ready to harvest when the tops wilt and turn yellowish-brown. Loosen the soil around the bulb with your hands or a trowel, then gently pull the bulb up.